1. Field of the Invention
The present invention relates to a thin-film capacitor such as the one having a large capacity and a low inductance, that is arranged, for example, in an electric circuit that operates at high speeds for bypassing high-frequency noise or for preventing fluctuation in the power-source voltage.
2. Description of the Prior Art
The modern trend toward fabricating electronic equipment in ever small sizes maintaining ever high functions, has strongly urged a reduction in the size and thickness of the electronic parts incorporated in the electronic equipment yet satisfying the requirement of operation at high frequencies.
In a high-speed digital circuit in a computer that must process large quantity of data at high speeds, in particular, the clock frequency in a CPU chip is as high as from 100 MHz to several hundred MHz, and the clock frequency on the bus among the chips is as high as from 30 MHz to 75 MHz even on a level of personal computers.
To cope with an increase in the degree of integration of LSI and an increase in the number of elements in the chip, furthermore, it has been attempted to lower the power-source voltage in order to suppress the consumption of electric power. Accompanying an increase in the operation speed, an increase in the density and a decrease in the voltage in the integrated circuits, passive parts such as capacitors must be produced in small sizes yet having large capacities and exhibiting excellent properties at high frequencies and for high-speed pulses.
In order to realize a capacitor of a small size having a large capacity, it is most effective to decrease the thickness of a dielectric held between a pair of electrodes into the form of a thin film. Forming a thin film is suited for lowering the voltage.
Problems that stem from high-speed operation of the ICs are more serious than problems that stem from decreasing the sizes of the elements. Among them, what is particularly important in removing high-frequency noise, which is the role of a capacitor, is a function for suppressing an instantaneous drop in the power-source voltage that occurs when the logic circuits are simultaneously changed over by instantaneously supplying energy stored in the capacitor. The capacitor having such a function is a so-called decoupling capacitor.
Performance required for the decoupling capacitor is how quickly an electric current can be supplied in response to a change in the current in the load unit which is quicker than a clock frequency. Therefore, the capacitor must reliably function in a frequency region of from 100 MHz to 1 GHZ.
In practice, however, the capacitor possesses a resistance component and an inductance component in addition to an electrostatic capacitance component. The impedance of capacitance component decreases with an increase in the frequency, and the inductance component increases with an increase in the frequency. As the operation frequency increases, therefore, a transient current supplied by the inductance of the element is limited, resulting in an instantaneous drop in the power-source voltage on the side of the logic circuit or in the occurrence of new voltage noise. As a result, an error occurs on the logic circuit.
In the modern LSIs, in particular, the power-source voltage has been lowered to suppress the consumption of electric power despite an increase in the total number of elements, and the width of allowable change in the power-source voltage has been narrowed. In order to minimize the width of change in the voltage during the high-speed operation, therefore, it is very important to decrease the inductance possessed by the decoupling capacitor itself.
The inductance can be decreased by three methods. A first method is to minimize the length of a current path. A second method is to form a current path in a loop structure and to minimize the sectional area of the loop. A third method is to divide the current path into n segments to decrease the effective inductance into 1/n.
The first method is effected by decreasing the size by increasing the capacity per a unit area, and is accomplished by preparing a capacitor element in the form of a thin film. In order to obtain a capacitor having a large capacity and favorable high-frequency characteristics, Japanese Laid-Open Patent Publication No. 94716/1985 discloses a dielectric in the form of a thin film having a thickness of not larger than 1 .mu.m.
The second method is to decrease the inductance by canceling a magnetic field formed by a current path by utilizing a magnetic field formed by another adjacent current path. This is done by directing a pair of electrodes forming a capacitor not in the same direction as much as possible or by directing the currents flowing into the electrodes not in the same direction as much as possible.
According to the third method, the divided capacitors are connected in parallel in order to decrease the inductance. As a capacitor of this kind, Japanese Laid-Open Patent Publication No. 211191/1992 discloses the one which utilizes a thin dielectric layer.
When a decoupling capacitor that can be mounted on a desired place is taken into consideration, however, the size that can be handled must not be smaller than about 0.5 mm.times.0.5 mm. Therefore, limitation is imposed on the first method of decreasing the inductance relying simply upon decreasing the thickness of the film and decreasing the size.
According to the second method, the positive and negative terminal electrodes must be arranged on the same end surface or in the directions at right angles with each other, which is disadvantageous from the standpoint of mounting.
The third method of division and parallel connection is advantageous when the capacitor is contained in the substrate without, however, offering freedom for mounting. The ordinary laminated-layer type capacitors are connected in parallel, but the directions of the currents are the same, and electric fields formed by the electrode currents are superposed. That is, the mutual inductance so increases that the effective total inductance is not decreased to a sufficient degree. Therefore, a second means must be employed in combination. As described above, the terminal electrodes bring about a problem from the standpoint of mounting.